This application is based on Japanese Patent Application No. 2002-118071 filed on Apr. 19, 2002, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates in general to an electroluminescent light-emitting device, and more particularly to an electroluminescent light-emitting device which is suitably used as an interior or exterior decorative or ornamental article, a signboard lighting device or toys, and which is arranged to emit a pattern of light which is desired by the user and which is defined by an electrically conductive ink that is applied by the user to the device in a pattern corresponding to the desired pattern of light.
2. Discussion of Related Art
There is known an electroluminescent (EL) light-emitting device including an electroluminescent light-emitting layer containing a suitable electroluminescent material, a front transparent electrode layer and a back electrode layer which are disposed on respective opposite sides of the light-emitting layer, so as to sandwich the light-emitting layer in the direction of thickness of the layers. By applying an AC voltage between the front and back electrodes, a local portion of the light-emitting layer is energized to emit a pattern of light which corresponds to a pattern in which the back electrode is formed. An example of the electroluminescent light-emitting device of this type is disclosed in JP-U-3034483. The light-emitting device disclosed in this publication takes the form of a thin plate, and is usable for various purposes, for instance, as a backlight device for a light-emitting display panel or decorative board.
In an electroluminescent light-emitting device as described above, a masking layer having a light-transmitting portion may be superposed on the front surface of the device over its entire area, so that the pattern of light emission from the device is defined by the light-transmitting portion. However, this device suffers from difficulty to change the pattern of light emission as desired by the user, and difficulty to prepare a mask having a shaded or half-tone portion. Thus, the conventional electroluminescent light-emitting device tends to suffer from a low degree of freedom in the pattern of light emission.
On the other hand, it has been proposed to form the back electrode in a desired pattern by applying a paste of an electrically conductive material by screen printing, for example. However, a screen or stencil for forming the back electrode is not easy and economical for the user of the device to manufacture.
The present invention was made in the light of the background art discussed above. It is therefore an object of the present invention to provide an electroluminescent light-emitting device which permits the user to change a pattern of light emission as desired in a simple manner.
The object indicated above may be achieved according to the principle of the present invention, which provides an electroluminescent light-emitting device having a light-emitting surface area, the device comprising: (a) an electroluminescent light-emitting layer containing an electroluminescent material; and (b) an electrode layer formed on one of opposite sides of the electroluminescent light-emitting layer, and including a first electrode and a second electrode which are formed in respective predetermined patterns and spaced apart from each other by spacing regions provided therebetween, in a direction parallel to a plane of the electrode layer, the first and second electrodes being electrically insulated from each other by the spacing regions, the electroluminescent light-emitting device having an exposed surface which is located on the other of the opposite sides of the electroluminescent light-emitting layer and to which an electrically conductive ink is applicable.
In the electroluminescent light-emitting device of the present invention constructed as described above, the electrically conductive ink is applied to the exposed surface of the device located on the side of the electroluminescent light-emitting layer remote from the electrode layer, while an AC voltage is applied between the first and second electrodes, so that there arises a flow of an alternating electric current between the first and second electrodes through the electrically conductive ink, whereby a local portion of the electroluminescent light-emitting layer which is located right below the applied electrically conductive ink emits light in a pattern formed by the ink in the light-emitting surface area of the exposed surface. Thus, the pattern of emission of light from the present electroluminescent light-emitting device can be easily formed and changed as desired, by the user of the electroluminescent light-emitting device.
According to one preferred form of this invention, the electroluminescent light-emitting device further comprises a top coating which covers one of opposite surfaces of the electroluminescent light-emitting layer which is remote from the electrode layer, the top coating having the exposed surface to which the electrically conductive ink is applicable.
In the electroluminescent light-emitting device according to the above-indicated preferred form of this invention, the user of the device applies the electrically conductive ink to the exposed surface of the top coating covering the electroluminescent light-emitting layer, while the AC voltage is applied between the first and second electrodes. The top coating is effective to protect the light-emitting layer, prevent permeation of the electrically conductive ink into the light-emitting layer, and facilitate the removal of the ink from the device when the ink is applied in a new pattern, for instance.
According to a first advantageous arrangement of the above-indicated preferred form of the invention, a surface area of the spacing regions of the electrode layer per unit area of the light-emitting surface area of the device is substantially constant throughout the light-emitting surface area. This arrangement assures a constant or uniform intensity of light emitted by the local portion of the electroluminescent light-emitting layer located right below the electrically conductive ink, irrespective of the location of this local portion (location of the ink), throughout the light-emitting surface area of the device. In other words, the present arrangement prevents a variation in the intensity of light emission from the electroluminescent light-emitting layer, which variation depends upon the specific location of the electrically conductive ink in the light-emitting surface area.
According to a second advantageous arrangement of the above-indicated preferred form of the invention, the electroluminescent light-emitting layer has a thickness within a range from 20 xcexcm to 50 xcexcm. This light-emitting layer assures a sufficiently high intensity of light emission. If the thickness is smaller than 20 xcexcm, the intensity of the electric field produced by the electroluminescent material is increased, but a mass of the electroluminescent material which emits light upon application of the voltage to the device is reduced. If the thickness is larger than 50 xcexcm, on the other hand, the above-indicated mass of the electroluminescent material is increased, but the intensity of the electric field produced by the electroluminescent material is reduced. Accordingly, the intensity of light emission is comparatively low where the thickness of the electroluminescent light-emitting layer is outside the range indicated above.
According to a third advantageous arrangement of the above-indicated preferred form of the invention, the electroluminescent light-emitting device a further comprises an electrically insulating reflecting layer which is interposed between the electroluminescent light-emitting layer and the electrode layer, to reflect light emitted by the electroluminescent light-emitting layer, back toward the light-emitting layer and the exposed surface of the top coating. In this arrangement, the light emitted by the light-emitting layer is reflected by the electrically insulating reflecting layer, back toward the light-emitting layer, thereby increasing the light-emitting efficiency of the present electroluminescent light-emitting device and the intensity of light emission from the device.
In the above-indicated third advantageous arrangement, the electrically insulating reflecting layer may be formed of a mixture of a power of a ferroelectric material and a resin binder in which the powder is dispersed. This reflecting layer appears substantially white, effectively functioning to reflect the light from the electroluminescent light-emitting layer, so that the intensity of light emission from the device is further increased. In addition, the use of the ferroelectric material having a high dielectric constant enables the reflecting layer to exhibit a sufficiently high dielectric constant, so that the intensity of the electric field produced by the electroluminescent material of the light-emitting layer is not significantly reduced by the electrically insulating reflecting layer interposed between the light-emitting layer and the electrode layer. Barium titanate or Rochelle salt may be used as the ferroelectric material.
In the above-indicated third advantageous arrangement, the electrically insulating reflecting layer may have a dielectric constant within a range of 30-100, preferably, 60-100. In this case, the reflecting layers interposed between the light-emitting layer and the electrode layer does not significantly reduce the intensity of the electric field of the light-emitting layer. It is noted that a material which gives the electrically insulating reflecting layer a dielectric constant exceeding 100 is expensive.
According to a fourth advantageous arrangement of the above-indicated preferred form of this invention, the top coating is formed of a synthetic resin capable of preventing permeation of the electrically conductive ink into the electroluminescent light-emitting layer. For example, the resin material of the top coating is selected so as to give the top coating a smooth surface for easy deposition and removal of the electrically conductive ink, and a high degree of resistance to permeation of the electrically conductive ink into the electroluminescent light-emitting layer. For instance, the resin material for the top coating is selected from among: tetrafluorinated ethylene; fluorine-containing synthetic resin such as fluoro-rubber; silicon resin such as silicon rubber; and polyester resin. In particular, the use of a fluorine-containing synthetic resin is advantageous for comparatively easy removal of the electrically conductive ink by wiping the surface top coating.
Preferably, the electrically conductive ink as applied to the top coating has a surface electrical resistance of not higher than 106 xcexa9/xe2x96xa1, and a relatively high degree of light transmittance. For instance, the electrically conductive ink consists of a mixture of a power of at least one electrically conductive material selected from among indium oxide, tin oxide, antimony and zinc oxide, and a solvent in which the powder is dispersed. This ink is effective to locally energize the electroluminescent light-emitting layer so as to emit light.